Re: [PATCH v7 04/22] liveupdate: luo_session: add sessions support

[Mike Rapoport]

On Sat, Nov 22, 2025 at 05:23:31PM -0500, Pasha Tatashin wrote:
> Introduce concept of "Live Update Sessions" within the LUO framework.
> LUO sessions provide a mechanism to group and manage `struct file *`
> instances (representing file descriptors) that need to be preserved
> across a kexec-based live update.
> 
> Each session is identified by a unique name and acts as a container
> for file objects whose state is critical to a userspace workload, such
> as a virtual machine or a high-performance database, aiming to maintain
> their functionality across a kernel transition.
> 
> This groundwork establishes the framework for preserving file-backed
> state across kernel updates, with the actual file data preservation
> mechanisms to be implemented in subsequent patches.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
>  include/linux/kho/abi/luo.h      |  71 +++++
>  include/uapi/linux/liveupdate.h  |   3 +
>  kernel/liveupdate/Makefile       |   3 +-
>  kernel/liveupdate/luo_core.c     |   9 +
>  kernel/liveupdate/luo_internal.h |  29 ++
>  kernel/liveupdate/luo_session.c  | 462 +++++++++++++++++++++++++++++++
>  6 files changed, 576 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/liveupdate/luo_session.c
> 

...

> +int __init luo_session_setup_outgoing(void *fdt_out)
> +{
> +	struct luo_session_header_ser *header_ser;
> +	void *outgoing_buffer;
> +	u64 header_ser_pa;
> +	int err;
> +
> +	outgoing_buffer = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
> +	if (IS_ERR(outgoing_buffer))
> +		return PTR_ERR(header_ser);

Should be 
		return PTR_ERR(outgoing_buffer);

Or, preferably, just drop outgoing_buffer and use header_ser everywhere.

> +	header_ser = outgoing_buffer;
> +	header_ser_pa = virt_to_phys(header_ser);
> +
> +	err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
> +	err |= fdt_property_string(fdt_out, "compatible",
> +				   LUO_FDT_SESSION_COMPATIBLE);
> +	err |= fdt_property(fdt_out, LUO_FDT_SESSION_HEADER, &header_ser_pa,
> +			    sizeof(header_ser_pa));
> +	err |= fdt_end_node(fdt_out);
> +
> +	if (err)
> +		goto err_unpreserve;
> +
> +	luo_session_global.outgoing.header_ser = header_ser;
> +	luo_session_global.outgoing.ser = (void *)(header_ser + 1);
> +	luo_session_global.outgoing.active = true;
> +
> +	return 0;
> +
> +err_unpreserve:
> +	kho_unpreserve_free(header_ser);
> +	return err;
> +}

...

> +int luo_session_deserialize(void)
> +{
> +	struct luo_session_header *sh = &luo_session_global.incoming;
> +	static bool is_deserialized;
> +	static int err;
> +
> +	/* If has been deserialized, always return the same error code */
> +	if (is_deserialized)
> +		return err;

is_deserialized and err are uninitialized here.

> +
> +	is_deserialized = true;
> +	if (!sh->active)
> +		return 0;
> +

...

> +/**
> + * luo_session_quiesce - Ensure no active sessions exist and lock session lists.
> + *
> + * Acquires exclusive write locks on both incoming and outgoing session lists.
> + * It then validates no sessions exist in either list.
> + *
> + * This mechanism is used during file handler un/registration to ensure that no
> + * sessions are currently using the handler, and no new sessions can be created
> + * while un/registration is in progress.

It makes sense to add something like this comment from luo_file.c here as well:

	 * This prevents registering new handlers while sessions are active or
	 * while deserialization is in progress.

> + *
> + * Return:
> + * true  - System is quiescent (0 sessions) and locked.
> + * false - Active sessions exist. The locks are released internally.
> + */

With those addressed:

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>


-- 
Sincerely yours,
Mike.